1. Field of the Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to active pixel sensors with kTC noise cancellation.
2. The Prior Art
In image sensors, an important source of noise is thermal reset noise, also known as kTC noise. Virtually all charge-coupled-device (CCD) imagers employ kTC noise cancellation because it is relatively easy to implement in CCD architectures, where the charge mode signal is brought to a single noise cancellation circuit (usually off chip). Elimination of kTC noise results in improved dynamic range, defined as the ratio between charge capacity and dark noise level. The dynamic range is the most often quoted figure of merit for CCD imager performance.
It is therefore not surprising that prospective CMOS imager vendors attempt to emulate this aspect of CCD imager performance. However, kTC noise cancellation is not as easy to implement in CMOS imagers as it is in CCD imagers since the read (charge-to-voltage conversion) circuit is inside the pixel sensor. The most common scheme used is referred to as correlated double sampling (CDS), in which the reset level is read out, then the signal level is read out, and then a subtraction operation is performed at the bottom of the column. (Dickenson, A., et al., “A 256X256 CMOS Active Pixel Image Sensor with Motion Detection,” 1995 DIGEST of TECHNICAL PAPERS, Institute of Electrical and Electronics Engineers, Volume Thirty-Eight, ISSN 0193-6530, Technical Paper 13.5, February 1995). However, this technique lends itself to the introduction of column pattern noise. A recent paper has described a once-per-die kTC noise canceling circuit. (Yonemoto K., et al., “A CMOS Image Sensor with a Simple FPN-Reduction Technology and a Hole Accumulated Diode,” 2000 DIGEST of TECHNICAL PAPERS, Institute of Electrical and Electronics Engineers, Volume Fourty-Three, ISSN 0193-6530, MP 6.1, February 2000). This approach will be difficult to implement for large arrays, due to the long settling time involved in reading the reference and signal levels. Furthermore, both of these cited prior-art approaches need a special charge-transfer active pixel sensor, such as a photo-gate active pixel sensor, that can store the signal charge while the reset charge is read out.